Digital Subtraction Angiography (DSA) is used in X-ray imaging to enhance visualization of a vessel structure in patient anatomy. In DSA, a mask image representing background information of a portion of anatomy in the absence of a contrast agent, is subtracted from an image of the same portion of anatomy with the vessel structure perfused by a contrast agent. The subtraction removes background detail and enhances the visualization of the vessel structure to improve catheterization and other procedures. It is useful, for example, to compare two DSA image sequence acquisitions in which one image sequence of a vessel structure is acquired before treatment and a second image sequence of the vessel structure is acquired after treatment to evaluate the effectiveness of treatment during interventional angiography such as a balloon catheterization. In order to perform this comparison, a point at which the two DSA image sequences are synchronized is selected. This is typically the time at which a contrast agent is first injected into a patient for each image sequence. However, the time of first contrast injection may vary between different image sequences so selection of the synchronization point is usually performed manually before the effectiveness of treatment may be evaluated based on side-by-side comparison of pre- and post-treatment image sequence acquisitions. Such manual selection is a slow, burdensome and error prone process. A system according to invention principles addresses these deficiencies and associated problems.